In temperate forests, soil respiration accounts for approximately half of total forest respiration and is regarded as the least well understood component of landscape carbon balance. Many research efforts have been aimed at determining the physical controls on total soil respiration and to that end, a wide variety of methods have been employed. The large-scale eddy covariance towers that are in operation at many key research sites are not able to resolve CO2 dynamics at the soil level and therefore surface based CO2 measurements are required. Subsurface approaches to soil CO2 monitoring are becoming increasingly popular. While requiring substantially more equipment than surface flux measurements, and limited in terms of spatial coverage and averaging, they offer significant advantages for resolving soil gas processes. Advantages include excellent vertical resolution of CO2 dynamics in the layered soil system, and the ability to estimate instantaneous CO2 production at specific depths. Such soil CO2 production estimates are helping to clarify temperature controls on soil respiration processes. This is especially true for shorter time scales when measured surface fluxes may often lag well behind actual CO2 production due to CO2 storage in the soil profile.
When used in conjunction with a diffusion model to determine instantaneous CO2 production, subsurface methods require good estimates of effective soil gas diffusivity (De). Concentration profiles of natural, injected 222Radon, or other tracers, have been used to determine diffusivity in the field. However, many researchers defer to empirically-derived approximations such as the Millington model or improved models requiring soil-specific input parameters. Unfortunately, diffusivity models tend to perform less well in some soils than in others.
There are several approaches that allow for laboratory testing of intact soil cores collected in the field. These have the advantage that soil gas diffusivity can be determined on a relatively fine scale that would be difficult to measure with 222Radon concentration profiles, and in highly organic substrates such as soil lifter that are not clearly dealt with in diffusivity model approximations. There remains, however, the potential for changes to soil physical properties (e.g. soil aggregation, compaction etc) that could have a large influence on resulting values. Consequently, it would be desirable to directly evaluate soil gas diffusivity in the field, as this would minimize potential problems with alteration to diffusivity in extracted soil cores and allows for field conditions to be monitored in-situ at the time of soil gas measurement.
Soil pore spaces are filled with varying quantities of air and water, and typical soil gas/vapour/liquid sampling techniques involve a buried probe that permits air, when present, to be withdrawn from the soil profile and contained for later analysis. Canadian patents CA 2215321 to Heller et al and CA 2072467 to Vollweiler illustrate such techniques. However, these methods can not make measurements without disturbing the soil air profile.
There is therefore a need for an apparatus and a method that allow continuous in-situ soil gas concentration and diffusivity measurements and that do not disturb the soil air profile.